High impact polystyrene polyblends (HIPS) comprising polystyrene having a rubber phase dispersed therein, as crosslinked rubber particles, are known. Historically, mechanical blends were prepared by melt blending polystyrene with raw rubber which was incompatible and dispersed as crosslinked rubber particles to reinforce and toughen the polymeric polyblend. More recently, HIPS polyblends have been prepared by mass polymerizing solutions of diene rubber dissolved in styrene monomer in batch reactors wherein the rubber molecules were grafted with styrene monomer forming polystyrene polymer grafts on the rubber along with polystyrene polymer in situ in the monomer. As the polystyrene-monomer phase increases during polymerization the grafted rubber phase inverts readily as rubber particles comprising grafted rubber and occluded polystyrene contained therein with said particles crosslinked to maintain the rubber particles as discrete particles dispersed in the polystyrene which forms a matrix phase of the HIPS polyblend.
Such HIPS polyblends conventionally contained rubber particles having an average particle size or diameter of about 0.5 to 1.0 microns to provide toughness yet small enough to insure good gloss for molding or sheet applications.
As such HIPS polyblends have moved in engineering applications even greater toughness is needed consistent with good gloss. Efforts to increase the toughness by adding more rubber have met with limited success in that the polymerization processes can only accommodate concentrations up to 15 to 20% and high levels of rubber increase cost and lower gloss. Efforts to increase the particle size of the rubber beyond about 1.0 microns to increase toughness as disclosed in U.S. Pat. No. 4,012,462 has met with limited success because of loss of gloss and flow properties.
It has been discovered that small amounts of larger sized rubber particles can be added to HIPS polyblends without loss of gloss or flow properties providing unexpectedly great increases in toughness beyond the proportionately small amounts of larger particles added to conventional HIPS polyblend having a rubber particle size averaging 0.5 to 1.0 microns.
It is hereby disclosed that the toughness of HIPS polyblends can be increased as much as 50% or more by the novel process of the present invention wherein HIPS polyblends are prepared having a bimodal rubber particle size distribution i.e., 70 to 95% of the rubber particles have an average particle size of about 0.5 to 1.0 microns and 5 to 30% of the rubber particles have an average rubber particle size of 2 to 3 microns.